This invention relates to a radiation detector for use in an apparatus, such as a positron type CT (computed tomography) apparatus or a single photon type ECT (emission computed tomography) apparatus, for detecting radiation (such as gamma rays) emitted from a radiative isotope (RI) administered to an object body and accumulated at a target location of interest to thereby obtain tomographic images of the RI distribution at the target location.
A radiation detector of this type is comprised of scintillators which emit light by absorbing gamma rays from an object body and photomultiplier tubes for converting light from the scintillators into electrical pulse signals. In prior art radiation detectors of this type, scintillators and photomultiplier tubes were provided in mutually corresponding one-to-one relationship. According to a recent method for improving resolution, however, a plurality of scintillators are coupled with a smaller number of photomultiplier tubes and the locations of incident gamma rays are determined from the output ratios of these photomultiplier tubes. For this reason, many types of radiation detectors have been proposed for properly distributing emitted light from scintillators among a plurality of photomultiplier tubes. The structure of some prior art radiation detectors will be explained next with reference to the drawings.
As shown in FIG. 5, a first example of prior art radiation detector (as described in Japanese Patent Publication Tokko 2-14666) is comprised of a scintillator group 1 with four scintillators 1.sub.1, 1.sub.2, 1.sub.3 and 1.sub.4 and two photomultiplier tubes 2.sub.1 and 2.sub.2. The two inside scintillators 1.sub.2 and 1.sub.3 of the scintillator group 1 are optically connected at their boundary with silicone grease or the like, but a reflective material is provided at the boundary between the pair of outside scintillators 1.sub.1 and 1.sub.2 and also between the pair of outside scintillators 1.sub.3 and 1.sub.4 such that each scintillator of the pairs is optically screened from the other and that the ratio of outputs from the photomultiplier tubes 2.sub.1 and 2.sub.2 will vary according to the location of incident gamma rays.
FIG. 6 shows a second example of prior art radiation detector (as described in Japanese Patent Publication Tokko 62-500957) comprised of a scintillator group 4 partitioned by many slits 3 and four photomultiplier tubes 5.sub.1, 5.sub.2, 5.sub.3 and 5.sub.4 optically connected to this scintillator group 4. A reflective material is buried inside each slit, and the slits near the peripheries of the scintillator group 4 are made deeper than those near the center such that locations of incident gamma rays can be distinguished.
FIG. 7 shows a third example of prior art radiation detector (as described in Japanese Patent Publication Tokkai 3-185385) comprised of a scintillator group 6 having a plurality of scintillators and four photomultiplier tubes 7.sub.1, 7.sub.2, 7.sub.3 and 7.sub.4 connected to this scintillator group 6. The surfaces of these scintillators through which they are optically connected are made rough and/or mirror-like and air layers are formed between these connecting surfaces. With this radiation detector, the locations of incident gamma rays can be distinguished because the optical transmissivity between the scintillators varies according to the surface conditions of the mutually opposite scintillators.
Each of the prior art radiation detectors described above has a problem. In the detector of the first example, since the scintillators are arranged one-dimensionally against the photomultiplier tubes, the number of photomultiplier tubes is rather large compared to that of the scintillators, resulting in an increased cost of production of the detector. Another problem of this detector is low resolution because only four different locations of incident gamma rays can be distinguished by using two photomultiplier tubes. As for the second example, the work on the scintillators to make slits therein and to uniformly fill them with a reflective material is both troublesome and difficult. If high resolution is desired, the slits must be provided at a small pitch and this makes the scintillators easy to damage. With the third example, the surface conditions of the scintillators must be varied in different ways, but such a surface processing is complicated and the production cost of the detector will be adversely affected.
The present invention has been accomplished in view of these problems, and its object is to provide a radiation detector which is both capable of identifying the locations of incident radiation with high resolution and easy to manufacture.